The present invention relates to a circulating type fluidized bed combustion apparatus used for burning solid fuel in boilers, incinerators and the like.
With the fluidized bed combustion apparatus, solid fuel can be burned at relatively low temperatures and therefore contents of nitrogen oxides contained in exhaust gases are less as compared with stationary combustion apparatus. Fluidized bed combustion apparatus are therefore widely used in boilers, incinerators and the like since desulfurization can be carried out in a fluidized bed and low-grade coal can be burned as fuel.
Fluidized bed combustion apparatus are, however, disadvantageous in that combustion proceeds mainly in a fluidized bed in the furnace and finely divided unburned particles are discharged out of the furnace. Moreover, although the desulfurization may be indeed carried out, a desulfurizing agent flows only in the fluidized bed and the desulfurization tends to be insufficient, resulting in a problem that the coefficient of utilization of the desulfurizing agent is still as low as CaO/S .apprxeq.4.
In order to solve the above and other problems, there has been devised and demonstrated a circulating type fluidized bed combustion apparatus in which bed materials are forced to circulate and fuel is burned in a jet-like or injected bed in a furnace which is blown upwardly at a high velocity. Such circulating fluidized bed combustion apparatus affords not only circulation of the bed materials but also that of the unburned fuel and other particles such as desulfurizing agent so as to burn them again and therefore, as compared with the above-described noncirculating type fluidized bed combustion apparatus, the combustion efficiency is as high as 99% and the coefficient of utility of the desulfurizing agent used is as high as CaO/S &lt;2. In addition, discharged amount of the nitrogen oxides can be also decreased since the temperature distribution in the furnace can be more uniformly maintained and the local combustion temperature can be decreased to some extent.
However, in the case of such circulating type fluidized bed combustion apparatus with higher gas velocities, there arises the problem that furnace height must be considerably high so as to attain a predetermined resident time. Furthermore, response to variation of load is not satisfactory because the volume of circulating particles cannot be controlled.
In view of the above, a primary object of the present invention is to provide a high-performance circulating type fluidized bed combustion apparatus with improved combustion efficiency and capable of readily responding to load varied over a wide range.
The above and other objects, effects and features of the present invention will become more apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings.